Sulfur is an essential element for cell survival, and its most readily available form in the environment is sulfate. There are three known families of sulfate transporters that are responsible for intracellular uptake of this ion. One family belongs o the ABC transporter family and is thought to function when the concentration of extracellular sulfate is low and energy is required for the process to occur. A second one belongs to the SulP class of major facilitator superfamily transporters. A third family are the products of the bacteril CysZ gene family. SulPs and CysZ are thought to be active when the concentration of sulfate in the environment is high enough to allow its entry inside the cell by a chemiosmotic driven mechanism. We have determined the crystal structure to 2.1 resolution of CysZ from Idiomarina loihiensis, a gram- negative deep-sea water bacterium and have established a functional assay for this protein. To the best of our knowledge this represents the first report ofa snapshot of a sulfate transporter at atomic resolution. The structure shows an unexpected and novel topology in which the protein assembles as a tight symmetric dimer across the plane of the membrane, so that the extracellular and intracellular sides of the molecule are the same. Two dimers pack against each other along hydrophobic helices protruding in and parallel to the membrane, to form a tetrameric assembly. The structure shows bound sulfate but the mechanism of transport has yet to be unveiled. We plan to perform experiments aimed at validating and understanding the remarkable architecture of CysZ (Aim 1). We also plan to understand how sulfate is transported across the membrane (Aim 2), and to characterize the factors that trigger an opening of the CysZ ion conductance pore (Aim 3).